1. Field of the Invention
This invention relates generally to a piston for internal combustion engines, and more particularly is directed to the piston-ring groove of a piston of aluminium alloy, which has good wear resistance.
2. Description of the Prior Art
In general, the piston-ring groove of a piston for an internal combustion engine is not only exposed to a high temperature, but also mechanically worn due to friction between the piston and a piston ring, so that blow-by is caused, and the consumption of lubricant increases. Thus, the life of the engine is shortened.
In case of a diesel engine in which abrasion on the piston-ring groove is very large, the piston is cast in special cast iron so as to reduce the abrasion. Further, it is disclosed in Japanese Laid Open Patent No. 53-31014 how to make aluminium alloy forming the piston body, penetrate into porous metal which is embedded around the piston-ring groove, when the piston is cast under high pressure.
Moreover, in Japanese Laid Open Patent Nos. 59-21393, 59-218341, and 59-212159, it is disclosed to increase the wear resistance of the piston ring groove reinforced by porous metal made of Fe, Ni, Cu or the like, in which hard intermetallic compound of porous metal and aluminium is produced by heat treatment.
The invnetors of this application found such facts that (1) the kind of the porous material influences in a process of forming an intermetallic compound of the porous material with aluminium, and (2) cracks are initiated in brittle portions of the formed intermetallic compound when the piston is repeatedly heated of long duration.
If a volumetric ratio V.sub.f of the porous metal ring is rather small, there is no detrimental effect on the piston even if the cracks are produced in the intermetallic compound. However, if the volumetric ratio of the porous metal ring increases, the cracks in the intermetallic compound will propagate in the aluminium alloy matrix at the boundary area between aluminium alloy matrix and the reinforced area with porous metal, resulting in the fall-out of the reinforced area from the piston body.
Moreover, there is another problem when the piston-ring groove is reinforced with the porous material. Even if the volume of the intermetallic compound is increased while keeping the volumetric ratio of the porous metal ring small, the wear resistance of the reinforced area is far less than the conventional Ni-resist ring carrier. In order to increase the wear resistance of the reinforced area with the porous metal to the acceptable level for heavy duty diesel engines, the volumetric ratio of the porous metal ring should be more than 20%. However, the reinforced area with such a high porous metal ratio can easily fall out from the piston, as mentioned already.